1. Field of the Invention
The present invention relates to a water-based fluorescent ink for providing fluorescence emission for the purpose of measurement or judgment in a visible light region. More specifically, the present invention relates to a water-based fluorescent ink that enables visual recognition of a recorded image under UV light irradiation but not under ordinary visible light, a water-based fluorescent ink that fluoresces under UV light but not under ordinary visible light, and an authenticity judging method using the above-mentioned ink.
2. Related Background Art
Recently, various characteristics are required water-based ink for further various applications, in addition to the conventional coloring purpose for recording an image such as characters, graphics, or the like on a recording material. In particular, use of an ink containing a fluorescence emitting component is applied not only for improving the visual color saturation of an obtained image, but also for amusement, and the purpose of the identification and classification, security, or the like. Water-based inks for various applications are now required. Such applications include, not only the formation of a beautiful color image, but also development of the technique for recording information such as characters, numbers, marks, and bar codes on a recording medium with a fluorescent ink that emits colored fluorescence under irradiation of ultraviolet light of an appropriate wavelength to provide information, e.g., security information, other than ordinary visual information. In particular, in a system for authenticity judgment (forgery prevention) or security information, a device reads intensity of fluorescence emitted from a fluorescent coloring material under irradiation of light of a reference wavelength (reference excitation wavelength, for example, 254 nm) for judgment or measurement.
Heretofore, various water-based fluorescent inks for the above-mentioned application have been proposed, discussed and reported to improve the water resistance and color properties including the fluorescence of the recorded image. Since a water-based ink can contain a water-soluble fluorescent material only in a small amount because of the concentration quenching phenomenon (a phenomenon that the fluorescence decreases as the content of the coloring material in the ink increase), it is difficult to enhance the fluorescence intensity. Moreover, conventional fluorescent inks contain a coloring material having a good dissolving property to prevent aggregation and association to obtain stronger fluorescent intensity, resulting in poor water resistance of the image.
On the other hand, when a compound that forms a salt with an aqueous fluorescent coloring material, e.g., a cationic compound that forms a salt with an acidic dye, is used to improve the water resistance, the water resistance is improved but the fluorescent property is deteriorated drastically.
To cope with this problem, Japanese Patent Application Publication No. H8-053640, for instance, proposes a water-based fluorescent ink containing an aqueous fluorescent dye in an emulsion or in capsuled form, whereby the water resistance is improved dramatically in comparison with conventional water-based fluorescent inks, and, even when the content of the emulsion or capsules containing the aqueous fluorescent dye in the ink is increased, aggregation or association of the aqueous fluorescent dye would not occur, and thus the fluorescent property can hardly be deteriorated. However, when the water or solvent content in the ink decreases, the ink is subject to thickening leading to insufficient sticking resistance. Moreover, when it is used for the ink jet recording method, the sticking resistance in the nozzle is insufficient. In the worst case, the nozzle is blocked with the ink. Furthermore, with the thermal ink jet recording method, koga deposits on the heater surface so that the ejection reliability is deteriorated drastically.